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GCP - IAM Privesc

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IAM

iam.roles.update (iam.roles.get)

If you have the mentioned permissions you will be able to update a role assigned to you and give you extra permissions to other resources like:

gcloud iam roldes update <rol name> --project <project> --add-permissions <permission>

You can find a script to automate the creation, exploit and cleaning of a vuln environment here and a python script to abuse this privilege here. For more information check the original research.

iam.serviceAccounts.getAccessToken (iam.serviceAccounts.get)

This permission allows to request an access token that belongs to a Service Account, so it's possible to request an access token of a Service Account with more privileges than ours.

You can find a script to automate the creation, exploit and cleaning of a vuln environment here and a python script to abuse this privilege here. For more information check the original research.

iam.serviceAccountKeys.create

This permission allows us to do something similar to the previous method, but instead of an access token, we are creating a user-managed key for a Service Account, which will allow us to access GCP as that Service Account.

gcloud iam service-accounts keys create --iam-account <name> /tmp/key.json

You can find a script to automate the creation, exploit and cleaning of a vuln environment here and a python script to abuse this privilege here. For more information check the original research.

Note that iam.serviceAccountKeys.update won't work to modify the key of a SA because to do that the permissions iam.serviceAccountKeys.create is also needed.

iam.serviceAccounts.implicitDelegation

If you have the iam.serviceAccounts.implicitDelegation permission on a Service Account that has the iam.serviceAccounts.getAccessToken permission on a third Service Account, then you can use implicitDelegation to create a token for that third Service Account. Here is a diagram to help explain.

You can find a script to automate the creation, exploit and cleaning of a vuln environment here and a python script to abuse this privilege here. For more information check the original research.

Note that according to the documentation, the delegation only works to generate a token using the generateAccessToken() method.

iam.serviceAccounts.signBlob

The iam.serviceAccounts.signBlob permission “allows signing of arbitrary payloads” in GCP. This means we can create an unsigined JWT of the SA and then send it as a blob to get the JWT signed by the SA we are targeting. For more information read this.

You can find a script to automate the creation, exploit and cleaning of a vuln environment here and a python script to abuse this privilege here and here. For more information check the original research.

iam.serviceAccounts.signJwt

Similar to how the previous method worked by signing arbitrary payloads, this method works by signing well-formed JSON web tokens (JWTs). The difference with the previous method is that instead of making google sign a blob containing a JWT, we use the signJWT method that already expects a JWT. This makes it easier to use but you can only sign JWT instead of any bytes.

You can find a script to automate the creation, exploit and cleaning of a vuln environment here and a python script to abuse this privilege here. For more information check the original research.

iam.serviceAccounts.setIamPolicy

This permission allows to add IAM policies to service accounts. You can abuse it to grant yourself the permissions you need to impersonate the service account. In the following example we are granting ourselves the “roles/iam.serviceAccountTokenCreator” role over the interesting SA:

gcloud iam service-accounts add-iam-policy-binding "${VICTIM_SA}@${PROJECT_ID}.iam.gserviceaccount.com" \
	--member="user:[email protected]" \
	--role="roles/iam.serviceAccountTokenCreator"

You can find a script to automate the creation, exploit and cleaning of a vuln environment here.

iam.serviceAccounts.actAs

This means that as part of creating certain resources, you must “actAs” the Service Account for the call to complete successfully. For example, when starting a new Compute Engine instance with an attached Service Account, you need iam.serviceAccounts.actAs on that Service Account. This is because without that permission, users could escalate permissions with fewer permissions to start with.

There are multiple individual methods that use _iam.serviceAccounts.actAs_, so depending on your own permissions, you may only be able to exploit one (or more) of these methods below. These methods are slightly different in that they require multiple permissions to exploit, rather than a single permission like all of the previous methods.

iam.serviceAccounts.getOpenIdToken

This permission can be used to generate an OpenID JWT. These are used to assert identity and do not necessarily carry any implicit authorization against a resource.

According to this interesting post, it's necessary to indicate the audience (service where you want to use the token to authenticate to) and you will receive a JWT signed by google indicating the service account and the audience of the JWT.

You can generate an OpenIDToken (if you have the access) with:

# First activate the SA with iam.serviceAccounts.getOpenIdToken over the other SA
gcloud auth activate-service-account --key-file=/path/to/svc_account.json
# Then, generate token
gcloud auth print-identity-token "${ATTACK_SA}@${PROJECT_ID}.iam.gserviceaccount.com" --audiences=https://example.com

Then you can just use it to access the service with:

curl -v -H "Authorization: Bearer id_token" https://some-cloud-run-uc.a.run.app

Some services that support authentication via this kind of tokens are:

You can find an example on how to create and OpenID token behalf a service account here.

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